The invention relates generally to the field of current sensors used for monitoring current flow in power systems. More specifically the invention relates to a self powered or remotely powered current sensor device providing an output signal.
Intelligent Electronic Devices (IEDs) comprise, but are not limited to digital power/energy meters, protective relays, power quality measurement devices, fault recorders or other devices capable of interfacing to electric power lines and calculating at least one power parameter. Power parameters include, but are not limited to rms current, rms voltage, kW, kVAR, kVA, frequency, harmonics, kWh, kVARh, kVAh, symmetrical components, etc.
Current transformers are used to monitor the current flowing in power system conductors. Generally, current transformers consist of two types. The first type is the closed (toroidal or rectangular) type. The second type is the clamp-on type. The closed type consists of a toroidal or substantially rectangular section of magnetic material with a “window” or opening through the middle. The current transformers have at least one secondary transformer winding that is wound around the material and through the window. A primary winding normally consists of a power line in a power system passing through the window that forms a single transformer turn. The winding ratio of the transformer is then the ratio of the primary to secondary turns. Clamp-on type current transformers are of substantially the same shape as closed type current transformers with the addition of a split in the magnetic material such that the transformer can be placed around the primary winding without having to “thread” the primary winding through the window. This allows installation of the clamp-on type current transformer on power system cables without disconnecting the power system cables from their source or load.
Standard current transformers suffer from errors in both ratio and phase shift mainly due to the magnetization current required to excite the magnetic material of the core. These effects limit the accuracy of the current transformer and dynamic range of current the transformers are able to sense. This is especially the case with clamp-on type current transformers due to the magnetic flux leakage caused by the split in the magnetic material.
An active or compensated current transformer circuit that corrects for such errors is described in U.S. Pat. No. 3,534,247 to Miljanic entitled “Current Transformer with Internal Error Compensation.” This circuit minimizes phase shift and ratio error during current transformation using a compensation amplifier. Powering the compensation amplifier from an additional current transformer is included in the active current transformer circuit. The presence of a separate powering current transformer means that additional wires are present beyond those providing the secondary current. This may make the device undesirable for installation in locations such as switchgear cabinets due to the high voltages present.
A second active current transformation approach is described in U.S. Pat. No. 4,841,236 to Miljanic et al. entitled “Current Ratio Device.” This approach provides additional isolation over the approach of the U.S. Pat. No. 3,534,247 through the inclusion of an isolated additional secondary winding which provides advantages for uses in high accuracy metrology applications. In general the accuracy of the approach of the U.S. Pat. No. 3,534,247 is more than adequate for most power system monitoring applications.
A self powered current monitor for monitoring current in an electric power system is described in U.S. Pat. No. 6,018,700 to Edel entitled “Self-Powered Current Monitor.” This circuit provides power for amplification circuitry, a microprocessor, etc. that is derived from the power line that is being monitored. The circuit includes a burden reducing circuit. The burden reducing circuit allows current monitoring to be performed with the same magnetic core that is powering the circuitry. The monitoring function of this circuit is not continuous or in alternate embodiments the burden of the power supply reduces the accuracy of the current transformation. Accordingly, this approach is difficult to use with accurate advanced power monitoring devices that continuously sample the current waveform in order to provide accurate power calculations and power quality functionality.
Intelligent Electronic Devices (“IEDS”) are available from multiple manufacturers. These IEDs commonly have current sensing inputs which accept current inputs from standard current transformers in the 5 Amp range. These current sensing inputs are typically two terminals. Two cables extend from the current transformers for connection to the terminals. Additional wiring of power supplies or separate powering cores as required by the previously described active current transformers is undesirable due to increased cost of installation, compliance with electrical codes, etc.